The present invention relates to a liquid crystal display device manufacturing method, and in particular, to a method of manufacturing a super twisted nematic (referred to as an "STN" hereinafter) type liquid crystal display device, a ferroelectric type liquid crystal display device and the like to be appropriately used as a color display means for a liquid crystal display device requiring a high cell gap accuracy in a thin type portable personal computer such as a notebook type personal computer.
In a series of processes for manufacturing a liquid crystal display device, there is a process of bonding together a pair of substrates (each referred to as a electrode substrate hereinafter) on which a display use electrode, an alignment film and so on are formed with interposition of a seal member provided in the vicinity of a display section of the liquid crystal display device and a spacer scattered or distributed uniformly inside the display region. In this substrate bonding process, a gap between the pair of substrates, namely, a cell gap is almost determined.
Conventionally, the control of the cell gap in the substrate bonding process has been performed by stacking a pair or a plurality of electrode substrates that have been bonded together as described above and concurrently pressurizing and heating these electrode substrates by means of a hot press machine. In general, for the seal member, a thermosetting resin including glass beads or the like operating as a spacer inside the seal is used, and glass beads or plastic beads are used for the spacer (referred to as an intra-cell spacer hereinafter) inside the display region.
When a nonuniformity exists in the cell gap, and even if the degree of nonuniformity is very small, it will appear as a display nonuniformity such as color shift when lighting the resulting completed liquid crystal display device. Therefore, it is very important to bond together the electrode substrates so that the cell gap becomes uniform. However, in the case of the method of concurrently performing the pressurizing and heating of the electrode substrates, the cell gap sometimes becomes nonuniform unless the pressurizing and heating are performed in a well-balanced manner throughout the entire substrate.
In view of the above, for example, the document of Japanese Patent Laid-Open Publication No. HEI 7-64101 discloses a way of varying continuously or in steps a pressurization force from a specified high pressure to a specified low pressure at a specified temperature before the attainment of the thermosetting temperature of the seal member by heating so as to obtain a uniform cell gap.
However, the prior art methods of controlling the cell gap by concurrently performing the pressurizing and heating of the electrode substrates, including the technique disclosed in the document of Japanese Patent Laid-Open Publication No. HEI 7-64101 have the following problems.
In the process of bonding together the substrates of the liquid crystal display device, the cell gap immediately after the bonding of a pair of electrode substrates is equivalent to the thickness (about 25 .mu.m) of a printed film of the seal member. When pressing the electrode substrates until the cell gap will come to have a specified value (6 .mu.m, for example), the pressing is performed with heating, and therefore, air inside the liquid crystal cell enclosed by the seal resin expands by a rise in temperature. Furthermore, in a case where the liquid crystal cell is a liquid crystal display device provided with a color filter employing a protective film, the amount of generation of gas from the protective film is also considered to be increased by the pressurizing with heating. Consequently, the internal pressure of the liquid crystal cell in the pressing stage increases to exert a bad influence on the finish of the seal, and a cell gap uniformity in the vicinity of the seal cannot be obtained, causing a display nonuniformity in the vicinity of the seal.
For the intra-cell spacer are widely used plastic beads which can follow the thermal expansion and contraction of the liquid crystal material and are easily elastically deformed. However, this plastic spacer is a polymer of divinylbenzene and an acrylic substance, and therefore, it causes a shape deformation when simultaneously receiving heat and pressure for a long time. This shape deformation has disturbed the uniformity of cell gap inside the display region and caused a circular display nonuniformity.
The influences of a finish accuracy of the cell gap in the vicinity of the seal and the accuracy of the cell gap uniformity inside the display region exerted on the display quality are especially significant particularly in the case of the STN type liquid crystal display device requiring a surface flatness of not greater than 0.05 .mu.m.
Furthermore, according to the aforementioned prior art method, the extension of the seal member through pressing and the hardening of the seal member are made to concurrently progress. Therefore, according to a multi-daylight press where a plurality of pairs (10 to 15 pairs) of electrode substrates are stacked, variations occur in the temperature rise of the substrate and the press pressure according to the stratal positions of the electrode substrates, and this consequently causes a problem that the cell gap finish differs depending on the stratal position.
In detail, the upper and lower electrode substrates located close to the surface plates are rapidly increased in temperature since they are located close to the heat source, while the electrode substrates located in the middle stratal positions located far apart from the surface plates are slowly increased in temperature. Then, the seal hardening progresses more rapidly at the seals that are rapidly increased in temperature, and this eventually lose the balance of pressurization.